Biochemical Society Transactions (2016) 44 1709–1716 DOI: 10.1042/BST20160219 The Borg family of Cdc42 effector proteins Cdc42EP1–5 Aaron J. Farrugia and Fernando Calvo Tumour Microenvironment Team, Division of Cancer Biology, Institute of Cancer Research, 237 Fulham Road, London SW2 6JB, U.K. Correspondence: Fernando Calvo ([email protected]) Despite being discovered more than 15 years ago, the Borg (binder of Rho GTPases) family of Cdc42 effector proteins (Cdc42EP1–5) remains largely uncharacterised and rela- tively little is known about their structure, regulation and role in development and disease. Recent studies are starting to unravel some of the key functional and mechanistic aspects of the Borg proteins, including their role in cytoskeletal remodelling and signal- ling. In addition, the participation of Borg proteins in important cellular processes such as cell shape, directed migration and differentiation is slowly emerging, directly linking Borgs with important physiological and pathological processes such as angiogenesis, neuro- transmission and cancer-associated desmoplasia. Here, we review some of these find- ings and discuss future prospects. Introduction The Rho GTPase family member Cdc42 regulates a diverse range of cellular functions including cyto- kinesis, cytoskeletal remodelling and cell polarity [1,2]. Like other Rho family members, Cdc42 cycles between two tightly regulated conformational states, a GTP-bound active state and a GDP-bound inactive state [3]. Activated Cdc42 exerts its functions by interacting with downstream effectors con- taining a Cdc42/Rac interactive binding (CRIB) motif [3,4]. Several Cdc42 effector proteins, including kinases and scaffolds, have been well characterised [5]; however, the functions of others remain rela- tively unknown. Still, elusive is the largely understudied Borg family of Cdc42 effector proteins, Cdc42EP1–5[6,7]. Borg proteins were simultaneously discovered by two independent groups as proteins that interact with the Rho GTPases Cdc42 and TC10/RhoQ [6,7]. Using a two-hybrid screen for interactors of TC10, the group of Ian Macara identified three clones containing identical CRIB motifs and flanking regions. Further analysis discovered the ability of these clones to bind active Cdc42 but not to Rac or Rho GTPases. These initial clones were subsequently extended to five putative clones based on sequence homology and their ability to bind Cdc42 in a GTP-dependent manner. They named this new family the Borg (binder of Rho GTPases) proteins (Borg1–5). One of these clones was the previ- ously identified MSE55 [8], which was renamed Borg5. MSE55/Borg5 had already been shown to contain a functional CRIB domain in its N-terminus and to be a non-kinase effector protein of Cdc42 capable of inducing F-actin-based protrusions [9]. Based on these characteristics, the group of Perter Burbelo used the amino acid sequence of MSE55/Borg5 to search for homologous sequences and Received: 30 July 2016 identified the same five clones [7]. They named the family ‘Cdc42 effector proteins’ or Cdc42EP/CEP, Revised: 15 September 2016 which led to the disparate nomenclature for each member (depending on the classification): MSE55/ Accepted: 19 September 2016 Cdc42EP1/Borg5, Cdc42EP2/Borg1, Cdc42EP3/Borg2, Cdc42EP4/Borg4 and Cdc42EP5/Borg3. For Version of Record published: clarity, here we use the HUGO gene nomenclature to refer to each member (i.e. Cdc42EP1 instead of 2 December 2016 MSE55/Borg5), but may use the term Borg to refer to the family. © 2016 The Author(s). This is an open access article published by Portland Press Limited on behalf of the Biochemical Society and distributed under the Creative Commons Attribution License 4.0 (CC BY). 1709 Biochemical Society Transactions (2016) 44 1709–1716 DOI: 10.1042/BST20160219 Figure 1. Cdc42EP/Borg family of Cdc42 effector proteins. (A) Schematic diagram showing the different domains in all the Borg proteins. (B) Direct alignments of the CRIB domains of murine Cdc42EP1–5, with conserved residues highlighted. (C) Direct alignments of the BH3 domain of murine Cdc42EP1–5, with conserved residues highlighted. Structure Borg proteins are relatively small in size, ranging from 150 amino acids (∼15.5 kDa) in Cdc42EP5 to 409 amino acids (∼39 kDa) in Cdc42EP1 (Figure 1A). No family member presents known enzymatically active domains, which suggest that they may exert their biological functions via structural or scaffolding activities. All Borg proteins are characterised by the presence of a highly homologous N-terminal CRIB domain (Figure 1B) [6,7]. In Borgs, the CRIB domain presents an extension at the C-terminus that may mediate its specific binding to active Cdc42 and TC10 but not to Rac1 [6,7], as well as determining part of the particular biological effects of these proteins. Following the CRIB motif, there is a well-conserved short domain that is unique to Borg pro- teins and that was defined as the Borg Homology (BH) 1 domain [6]. The central and C-terminal parts of the proteins are more divergent, but still present two additional well-conserved regions termed BH2 and BH3. The BH2 domain is absent in the smallest Borg, Cdc42EP5, whereas the BH3 domain is common to all Borgs. The BH3 domain has a central location in Cdc42EP1 and Cdc42EP4, whereas it is localised at the C-terminal parts of Cdc42EP2, Cdc42EP3 and Cdc42EP5 (Figure 1C). The BH3 domain is the only BH domain with a defined molecular role and has been shown to be necessary and sufficient for the specific binding of Borgs to septins [10] (see Box 1). Contrary to the BH3 domain, the roles of the BH1 and BH2 domains are still to be defined. 1710 © 2016 The Author(s). This is an open access article published by Portland Press Limited on behalf of the Biochemical Society and distributed under the Creative Commons Attribution License 4.0 (CC BY). Biochemical Society Transactions (2016) 44 1709–1716 DOI: 10.1042/BST20160219 Box 1. The septin cytoskeleton Septins comprise a large family of proteins (13 genes and >30 protein isoforms in humans) with ubiquitous and tissue-specific expressions [12]. Septins are GTP-binding proteins that participate in numerous cellular processes, including cell division, cell polarity and cytoskeletal organisation [13,14]. Septins can dimerise and assemble into oligomers, such as the SEPT2/6/7/9 polymer, which is the best characterised in mammalian cells [15,16]. Septin oligomers can assemble into higher-order structures such as filaments, bundled filaments and rings, which can serve as scaf- folds and diffusion barriers that control the localisation of cellular proteins [17]. The septin fila- mentous network can associate with cell membranes and has been shown to co-localise with actin, tubulin and other cytoskeletal components [18,19]; thus, septins are increasingly recog- nised as a novel component of the cytoskeleton [13]. In addition to the improved understanding of the biochemical properties and functions of septins, a growing number of studies are highlight- ing the relevance of septins in the development and physiology of specific tissues and organs [12]. Outside the CRIB and BH domains, sequence divergences emerge. The central regions of Cdc42EP1, Cdc42EP2 and Cdc42EP5 present a proline-rich domain; in addition, Cdc42EP1 contains several heptad repeats that follow the BH3 domain [10]. More recently, it has been described that Cdc42EP3 presents a puta- tive actin-binding region in its central region, C-terminally to the BH2 domain (Figure 1A)[11]. This region presents high homology to the actin bundling region of Anillin. This particular actin-binding region of Cdc42EP3 is not conserved among Borg proteins; however, other Borg members may bind and/or modulate the actin cytoskeleton by alternative mechanisms or via distinct domains. Mode of action At cellular and molecular levels, the function of Borg proteins remains to be clearly defined, but studies thus far indicate relevant roles in cytoskeletal rearrangement, as summarised in Table 1. As with other Cdc42 effector proteins, Borg proteins were initially linked to the regulation of cell shape and early gain-of-function experiments described roles in inducing pseudopodia and F-actin-containing structures in fibroblasts [7,9] and Table 1 The major interactions and functions of Borg proteins CDC42 effector Binding protein Interactions partners Reported functions Key references Cdc42EP1 Cdc42, TC10 SEPT2/6/7 Cell shape regulation, [6,7,9,10,20,21,33,34] (Borg5, (RhoQ), aPKC, actomyosin contractility, cell MSE55) ERK2, multiple sites migration of endothelial cells identified by and trophectoderm cells, proteomic analysis directed and persistent migration, angiogenesis Cdc42EP2 Cdc42, TC10, ERK2, SEPT2/6/7 Cell shape regulation and [6,7,10,20,32,33,37] (Borg1) ERK3, MK5 regulator of myogenesis Cdc42EP3 Cdc42, TC10, ERK3, SEPT2/6/7 Cell shape regulation, [6,7,10,11,20,31,32] (Borg2) MK5 F-actin actomyosin contractility and pathological fibroblast activation Cdc42EP4 Cdc42, TC10, PKCα SEPT2/6/7, Cell shape regulation, [6,7,10,20,23,24] (Borg4) ARHGEF17 filopodia formation and (TEM4) mammary cell migration Cdc42EP5 Cdc42, ERK3, MK5 SEPT2/6/7 Cell shape regulation and [6,7,10,20,32] (Borg3) lamellipodium formation © 2016 The Author(s). This is an open access article published by Portland Press Limited on behalf of the Biochemical Society and distributed under the Creative Commons Attribution License 4.0 (CC BY). 1711 Biochemical Society Transactions (2016) 44 1709–1716 DOI: 10.1042/BST20160219 Figure 2. Links between Borg proteins and cytoskeletal regulation and cell shape. Cartoon showing the cellular localisations of Borg proteins. See text for details. epithelial cells [7]. Shortly after, Joberty et al. [10] elegantly demonstrated that Cdc42EP5 and Cdc42EP2 can bind septins via their BH3 domain and induce septin filament bundling. Further characterisation demonstrated that Cdc42EP5 binds specifically to SEPT6/7 heterodimers or SEPT2/6/7 trimers, but not to septin monomers [20]. This septin-binding property has since been extended to the rest of the members of the family [11,21– 23].